Printed circuit boards, backplanes, midplanes, printed wiring boards, flex circuits, rigid flex-circuits, multi-chip modules (MCM), interposers and the like are herein referred to collectively as “PCBs”.
A via structure typically provides a conductive path between conductive layers in the z-axis direction (orthogonal to the x-y plane of a PCB). Via holes are formed by a variety of techniques including but not limited to laser drilling, mechanical drilling, and techniques based on photo definition. Via holes are subsequently partially or wholly filled or coated with a conductive material, usually metal. Such via structures may be blind, buried, through-hole and may or may not include pads on the conductive layers, as is well known to those skilled in the art of PCB design.
Sensitive components on a printed circuit board can be damaged by transient occurrences of electrostatic discharges (ESD). An ESD is characterized by a rapid rise in the order of tens of kilovolts in a few picoseconds, for example. Other transient phenomena with lower peak voltage levels and slower rise-times can also cause damage to the printed circuit board. For example, a sudden rise in voltage can be caused by a poorly grounded soldering iron, or a power switching relay, or a lightning strike on telecommunication lines that are connected to the printed circuit board. The term “transient” as used herein encompasses not only ESD events but any phenomena, of short duration, that directly or indirectly induces voltages and currents into a printed circuit board and where the amplitudes of such voltages and currents are high enough to cause degradation or failure of the electronic components on the printed circuit board.
FIG. 1A is a schematic that illustrates a printed circuit board 102 protected by conductive guard rings 104. Printed circuit board (PCB) 102 has a length L and a width W. In FIG. 1A, conductive guard rings 104 (only one of which is visible in FIG. 1) are added to the periphery of each outer layer of PCB 102 and one or more discrete transient protection devices can be attached to PCB 102. The guard rings 104 are attached to the chassis ground at the location where I/O connectors 106 are mounted to PCB 102. Typically, when a person picks up a PCB, the person will initially touch the periphery of the PCB. By positioning guard rings 104 along the periphery of PCB 102, guard rings 104 re-direct undesired transient currents to chassis ground. Thus, detrimental currents are not allowed to flow to transient sensitive components on PCB 102. However, guard rings fail to protect interior surfaces 112 of PCB 102. Another form of transient protection is the use of discrete transient protection devices.
Discrete transient protection devices such as discrete transient protection devices 108 can be attached to PCB 102 at the location where signal and/or power lines enter PCB 102, such as connector 106. However, discrete transient protection devices consume valuable real estate on the PCB. For example, U.S. Pat. No. 6,657,532 discloses discrete over-voltage protection components made of a thin layer of neat dielectric polymer or glass positioned between a ground plane and an electric conductor. U.S. Pat. No. 6,657,532 also discloses discrete over-voltage protection components having multi-layers of variable voltage material. Another non-limiting example of a discrete transient protection device is a resettable polymeric-positive-temperature-coefficient (PPTC) device or a voltage switchable dielectric material (VSDM). Like fuses, PPTC devices help protect circuitry from overcurrent damage. However, discrete PPTC devices consume valuable real estate on the PCB.
Other forms of transient protection include on-chip transient protection devices 110, such as zener diodes, for example. However, such on-chip transient protection devices do not have sufficient capacity to effectively dissipate large transient events. Both discrete and on-chip transient protection devices often have excessive amounts of intrinsic capacitance that makes such devices unsuitable for use in high speed applications. The primary protection mechanism of both discrete and on-chip transient protection devices is through the conversion of undesired transient energy into heat. Thus, large transient magnitudes and/or repeated exposure to large transient magnitudes are likely to result in over-heating that in turn results in performance degradation of such devices.
FIG. 1B is a cross section 150 of the PCB 102 of FIG. 1A taken at 1B. Cross section 150 shows that the PCB comprises multi-layers 160 of material. Cross section 150 also shows guard ring 104, on-chip transient protection device 110, connector 106 and discrete protection devices 108.
According to certain embodiments of the invention, a voltage switchable dielectric material can be used as transient protection material. In the past, voltage switchable dielectric material was used to make an insulating substrate that can be made conductive. When conductive, the voltage switchable dielectric material is amenable to electrochemical processing such as electroplating for making conductive traces. Such a method is disclosed by U.S. Pat. No. 6,797,145. Thus, U.S. Pat. No. 6,797,145 discloses the use of voltage switchable dielectric material as an insulating substrate that can be made conductive for making conductive traces.
Thus, in view of the foregoing, an effective form of transient protection is needed.